chemical thermodynamics of neptunium and plutonium - U.S. ...

A. Discussion of selected references 705[84MUL/GOR]In the course of this kinetic work, Mulac et al. measured the redox potential of the actinide(VI)/actinide(V)couple by cyclic voltammetry in Na 2 CO 3 aqueous solution forAn = U, Np and Pu. The actual Na 2 CO 3 concentration was not clearly reported, but itwas probably 0.05 M. There is no indication of the junction potential. For these reasons,this review did not use the data reported in this work. The actinide(VI)/actinide(V)couple was written as AnO 2 (CO 3 ) 4−3 /AnO 2(CO 3 ) 5−3in this publication, which is correctin concentrated Na 2 CO 3 aqueous solution; however, in 0.05 M Na 2 CO 3 aqueoussolution the dissociation of the actinide(V) limiting complex should probably not beneglected. A more detailed re-interpretation is given in [98VIT/CAP].[84NAS/CLE]Nash and Cleveland’s primary aim was to explain, with the help of the thermodynamicsof fluoride complexation of Pu(IV), a plutonium solubility as high as 10 −7 Mina groundwater of pH = 9.3 that contained 2.7 mM fluoride. Their interest lay also inchecking if temperature variations could have a significant impact on fluoride complexation.They used a solvent extraction technique with the extractant HTTA in benzene at25 ◦ C. In one experimental set, the ionic strength of the aqueous solution was kept constantat I = 2 M, maintained by HClO 4 , and the 1:1 and 1:2 formation constants weredetermined at 1, 7.5, 15 and 25 ◦ C. In another experimental set the background electrolytewas a mixture of 1 M NaClO 4 and1MHClO 4 , and the plutonium concentrationwas varied by a factor of about three. The experimental procedure is satisfactory. Theauthors reported enthalpies of the equilibriaPu 4+ + qHF(aq) Å PuF 4−qq + qH + (A.60)for q = 1 and 2, from the determinations between 1 and 25 ◦ C: r H m (A.60, q =1, I = 2M) =−(3.08 ± 0.32) kJ·mol −1 and r H m (A.60, q = 2, I = 2M) =−(14.4 ± 1.8) kJ·mol −1 . The Van’t Hoff plots ([84NAS/CLE], Figure 2) show aslight scattering of the data, especially for the 1:2 complex. Since the authors assigneddifferent uncertainties to each constant, we prefer to use these uncertaintiesin a weighted linear regression according to the outline in Appendix C.3. We obtain r Hm ◦ (A.60, q = 1, I = 2M) =−(3.13 ± 2.15) kJ·mol−1 and r Hm ◦ (A.60, q =2, I = 2M) =−(13.4 ± 4.9) kJ·mol −1 . These constants are accepted. We assume thattheir ionic strength dependence is small, and that they are also valid at I = 0.The authors corrected the resulting constants to I = 0 by estimating the activitycoefficients. We prefer to use the specific ion interaction equation for this correction,and we use the constant determined at 25 ◦ Cin2MH + , as well as the average fromthe three determinations in 1 M H + , in our evaluation of log ∗ 10 β1 ◦ (A.60, q = 1) andlog ∗ 10 β2 ◦ (A.60, q = 2). We assign an uncertainty of ±0.20 to all values.[84PER/SAP]This paper reports solubility boundaries with respect to the Pu(IV) concentration inaqueous solution and pH. The boundary position, if interpreted with respect to Pu 4+ as